What is the second most abundant gas in the Earth's atmosphere, accounting for 21%?

Oxygen is the second most abundant gas in Earth's atmosphere, comprising about 21% of the atmosphere's composition. This gas is vital for life as it is essential for respiration in most living organisms. The process of photosynthesis, performed by plants and some microorganisms, produces oxygen and contributes to its abundance. The significant presence of oxygen supports various ecosystems and is crucial for maintaining life on the planet.

While nitrogen is the most abundant gas, making up about 78% of the atmosphere, that does not affect the status of oxygen as the second most abundant gas. Carbon dioxide, although important for various biological and chemical processes, is present in much smaller quantities, roughly 0.04%. Helium, which is a noble gas, is also found in trace amounts and does not contribute significantly to the overall composition of the atmosphere. Thus, oxygen's role and percentage solidify its position as the second most prevalent atmospheric gas.